A mystery attacker ambushed a mild-mannered paralegal as he left a Manhattan strip club – delivering a fatal punch to the back of his head for no apparent reason, police said yesterday.

Investigators have no idea why Neil Eddleson, 39, was attacked outside New York Dolls in TriBeCa on Tuesday evening – and it’s possible the bizarre incident was a random assault.

“He was in the wrong place at the wrong time,” the victim’s cousin, Mitchell Sudman, said at the Sheepshead Bay, Brooklyn, home where Eddleson lived with his parents.

Eddleson had spent a few hours drinking Sambuca with a close friend at the upscale topless bar, where he was a regular patron.

He and his pal, Monty Seid, had left the bar and were saying goodbye on the corner when two men appeared behind them, cops said.

One ran up from behind and socked Eddleson in the back of the head, knocking him to the pavement, where he struck his head.

Seid said he walked over to the assailant, who was mumbling something.

“I said, ‘What are you doing?’ Then I saw my friend was bleeding. That was the end of it,” he said.

“I wasn’t afraid for myself. I didn’t think he was a crazy guy. He didn’t look like he wanted to hit me. Then he ran off.”

Eddleson was taken to New York Downtown Hospital, where he was pronounced dead at 10 a.m. An autopsy showed he died from skull fractures and brain injuries.

“I didn’t think he’d die from just one hit,” Seid wept. “It’s just not right.”

Investigators said it appears the attack was not a robbery attempt. Eddleson, who friends said was a paralegal in the federal courts, had $500 in his wallet when he died.

Police believe the killer may have been at the club, which was holding its holiday party, but cops said there was no report of any trouble inside.

They were hoping to create a police sketch of the attacker, and they were reviewing tapes from video cameras in the area.

New York Dolls manager Frank Felice said Eddleson was a once-a-week customer who usually came in with his friends and left after about an hour.

“It’s a shame,” he said. “He was a very nice guy, a very quiet guy. He never bothered anybody.”

Eddleson’s family was making arrangements for a funeral today.

“I’m numb,” said his father, Jerry, a retired electrical-design engineer. “He was a good person.”

His mother, Mildred, was overcome with emotion, crying out, “They need to find who did this.”

Family members were supposed to gather tomorrow for a Hanukkah celebration. Instead, they will be in mourning.

“It’s unbelievable,” said Brian Greenberg, a family friend for 10 years. “He’s one of the quietest, nicest guys you’ll ever meet. He never bothered anybody. He never got in trouble. He was the easiest person to talk to.”